refactor: migrate to DDD architecture with Dishka DI

Complete architectural refactoring from simple MVC to Clean Architecture/DDD pattern:

Domain Layer:

- Add entities (Post, BaseEntity) with business logic

- Add value objects (Title, Content, Slug) with validation

- Add repository interfaces (PostRepository)

- Add domain exceptions

Application Layer:

- Add use cases (CreatePost, GetPost, UpdatePost, DeletePost, ListPosts, PublishPost)

- Add DTOs for data transfer

- Add TransactionManager interface

Infrastructure Layer:

- Add SQLAlchemy models and async database connection

- Add SQLAlchemyPostRepository implementation

- Add Dishka DI container with providers

- Add error handlers and middleware

Presentation Layer:

- Add FastAPI routes with Dishka integration

- Add Pydantic schemas

- Add dependency injection using FromDishka[T]

Other Changes:

- Remove old flat structure (api/, common/, core/, modules/)

- Add hatchling build system for package scripts

- Add blog CLI command

- Update AGENTS.md with new architecture docs

- All 48 tests passing, mypy clean, ruff clean

app/application/use_cases/__init__.py

@@ -0,0 +1,17 @@
+"""Use cases."""
+
+from app.application.use_cases.create_post import CreatePostUseCase
+from app.application.use_cases.delete_post import DeletePostUseCase
+from app.application.use_cases.get_post import GetPostUseCase
+from app.application.use_cases.list_posts import ListPostsUseCase
+from app.application.use_cases.publish_post import PublishPostUseCase
+from app.application.use_cases.update_post import UpdatePostUseCase
+
+__all__ = [
+    "CreatePostUseCase",
+    "GetPostUseCase",
+    "UpdatePostUseCase",
+    "DeletePostUseCase",
+    "ListPostsUseCase",
+    "PublishPostUseCase",
+]